The present invention relates to an assembly of a light source and an optical detector, each using a vertical cavity surface emitting laser (VCSEL) of a stack structure formed on a single substrate.
A VCSEL emits light vertically from a stack of semiconductor materials deposited on a substrate, unlike an edge emitting laser. Properties of the VCSEL include circular light output and single longitudinal mode operation. A light source, and an optical detector for detecting an error signal from an optical signal of light emitted from the light source can be integrated in this VCSEL. These advantages makes the VCSEL attractive for optical applications such as the light source for an optical pick-up.
However, since the lower surface of the VCSEL is combined with a semiconductor substrate while light is vertically emitted from the upper surface thereof, it is difficult to install an optical detector for monitoring.
To overcome this problem, a feed-back mechanism for the VCSEL is disclosed in U.S. Pat. No. 5,285,466.
In this mechanism, vertical light emission of a first VCSEL being a light source is controlled by using non-directional lateral spontaneous light thereof. The lateral spontaneous light is received in a second VCSEL provided around the first VCSEL. A signal detected from the received light is electrically fed back to the light source portion to control light emission of the light source. That is, the second VCSEL is used as an optical detector for receiving the lateral spontaneous light by applying no power or reverse biased voltage to the second VCSEL.
The structure and operation of a conventional assembly of a VCSEL light source and a VCSEL optical detector is shown in FIG. 1. The conventional assembly of a VCSEL light source and a VCSEL optical detector is comprised of a semiconductor substrate 10, a layered VCSEL light source 12 integrated on semiconductor substrate 10 for emitting light by application of a forward biased voltage, and an annular optical detector 14 surrounding light source 12 for absorbing lateral spontaneous light emitted therefrom. Optical detector 14 absorbs the spontaneous light emitted from light source 12, converts the light into an electrical signal, feeding it back to an electrode of light source 12, to control light emission. A, reverse biased voltage is applied or no power is supplied to optical detector 14.
However, since annular optical detector 14 defines a very short light path, much of the light absorbed into the inner surface thereof penetrates through the outer surface thereof, and is not used to generate signal thereby lowering its light absorption level. As a result, it is very difficult to obtain light intensity large enough to detect a signal for controlling light emission of light source 12.